WO2006021726A2

WO2006021726A2 - Switched-mode power supply regulation

Info

Publication number: WO2006021726A2
Application number: PCT/FR2005/050646
Authority: WO
Inventors: Alain Bailly
Original assignee: Stmicroelectronics Sa
Priority date: 2004-08-06
Filing date: 2005-08-04
Publication date: 2006-03-02
Also published as: US20070200610A1; FR2874138A1; US7825645B2; EP1800390A2; WO2006021726A3; CN101167240A

Abstract

The invention concerns a circuit (30) for detecting an overload in a load supplied by a switched-mode power supply, comprising: a first comparator (25) of a first voltage based on the supply voltage of the load relative to a first threshold (VFB), supplying a regulating signal (CT) to a pulse generator (6) controlling the switched-mode power supply; a second comparator (31) of a second voltage relative to a second threshold (VOLV), supplying a signal (OVL) indicating the presence of an overload; and means (C33, 34, 35, M35) for automatically controlling said second voltage by a third threshold (VINI) lower than the second and higher than the first, and for deactivating the second comparator as long as said automatic control is maintained.

Description

the

REGULATION OF A CUTTING FEED

Field of invention

The present invention relates to the field of switching ALIMEN ^¬ tations and more particularly the control of a switching power supply from a measurement of the output voltage supplied to the secondary of this power supply.

The present invention applies more particularly to a switching power supply of which the inductive element is part of a transformer to isolate the voltage ALIMEN ^¬ tation of the charging voltage supplied to the switching power supply. An optocoupler is then necessary in order to respect the isolation barrier and the absence of a common ground, also for the regulation signal. Presentation of the prior art

FIG. 1 very schematically shows, partially in the form of blocks, an example of a conventional switching power supply of this type. In this example, the power source is an AC voltage Vac (for example, the mains voltage from the electrical distribution network). The voltage Vac is rectified by a diode bridge 1 (for example, double alternation) whose rectified output terminals 2 and 3 are connected by a capacitor Cp, at the terminals of which a smoothed DC voltage is present. This voltage is applied across a primary winding 4p of a transformer 4 being cut by a switch 5 in series with this winding. The switch 5 is controlled by a pulse train provided by a pulse width modulation circuit (PWM). It can also be a modulation of the frequency of the pulses.

Secondary side (winding 4s) of the transformer 4, a diode D in series with a capacitor Cs is connected across the winding 4s. The capacitor Cs provides a charge voltage Vout of a load 7 (Q) between output terminals 8 and 9 of the switching power supply. Information on the voltage Vout is also taken (for example at the terminals 8 and 9) to a measuring circuit (MES) constituting a control loop of the closing periods of the switch 5 as a function of a supply voltage setpoint of the charge Q. The circuit 10 controls a ^photodiode PD of an optocoupler 11 whose phototransistor PT is connected to a regulation circuit 12 (REG) responsible for providing, in block 6 of generating the pulse train, at least a first setpoint signal CT. A second OVL signal for detecting a possible overload, that is to say a too high current demand by the load 7 is also supplied to the circuit 6 by the circuit 12.

The example of FIG. 1 is that of a so-called "flyback" converter in which the energy is transferred from the primary to the secondary circuit during the periods when the switch 5 is open. The invention is however not limited to this type of converter and also applies to forward-type converters in which the energy transfer takes place during the closing periods of

The cutting switch.

Figure 2 shows a typical example of a circuit

12, whose output terminals 26 and 21 supply, to a pulse train generating circuit 6, respectively a control signal CT and an OVL detection signal. overload. This circuit comprises, in series with the ^{phototransistor} PT between a terminal 23 for applying a direct supply voltage Vcc and the ground 24 on the secondary side, a capacitor C12. Since the circuit 12 is generally in the form of an integrated circuit, the midpoint of this series ^associa¬ tion is connected directly to an input terminal 20 of the control signal FB (that is to say of connection of the PT phototransistor transmitter). An analog comparator 25 (differential amplifi¬) has its non-inverting input connected to the terminal 20 and its inverting input which receives a fixed reference voltage Vpg conditioning the output voltage Vout of the converter. The output of the comparator 25 controls an interrupter M (for example, a MOS transistor) which connects the terminal 20 to the terminal 26 for supplying a control current (signal CT) to the circuit 6 (FIG. 1). A constant current source 27 also connects the terminal 20 to the ground 24.

The role of the comparator 25 is to regulate the potential of the terminal 20 (hence the emitter of the phototransistor) to the value of the voltage V _FB . When the needs of the load decrease, the voltage

Vout tends to increase. The circuit 10 (FIG. 1) then controls the emitting diode PD which increases the base current of the phototransistor PT. Assuming the capacitor 12 loaded

(steady state), the increase of the current in the phototransistor PT increases the potential of the non-inverting input of the comparator 25 since the current source 27 can not discharge more current (constant current). . The output voltage of the comparator 25 decreases and the current in the transistor M increases. The current on the terminal 26 constituting the current control signal of the generation circuit 6 of the pulse train increases and is interpreted by the circuit 6 to, in the example of Figure 1, decrease the closing periods of the switch 5, in order to accumulate less energy and thus reduce the output voltage Vout. the the the

If the load requires more energy, the voltage Vout tends to drop. This decrease results in a decrease of the current in the optocoupler which tends to lower the voltage of the terminal 20. In fact, the capacitor C12 discharges into the current source 27, which causes an increase in the output voltage of the comparator 25 and a decrease in the conduction of the transistor M. The current on the terminal 26 decreases and is interpreted by the circuit 6 to increase the switch closure periods 5 to accumulate more energy and increase the output voltage Vout.

The circuit 12 comprises a second comparator 28 for overload detection. This comparator has its inverting input connected to terminal 20 and its non-inverting input which receives a voltage VQVL constituting an overload threshold. The output of comparator 28 is connected to terminal 21 which supplies an OVL overload detection signal to circuit 6.

In the presence of an overload, the higher current demand demanded at the output causes a sudden reduction of the current in the optocoupler which will even go out. Without overload detection, terminal 26 would provide a CT signal requesting circuit 6 to provide even more power. However, in case of overload, it is advisable conversely to stop supply ^¬ ture energy secondary.

The role of the comparator 28 is to detect when the comparator can no longer maintain the terminal 20 at the voltage V _FB by the regulation. The voltage VQVL ^is chosen lower than the voltage V _FB and the comparator 28 switches when the discharge of the capacitor C12 in the current source 27 is such that the terminal FB reaches the threshold VQY ^. In normal operation, the regulation does not allow the capacitor C12 to discharge sufficiently, preventing the triggering of the comparator 28. In contrast to the comparator 25, the compa ^¬ tor 28 is generally a comparator output in all or nothing. In a control circuit as illustrated in FIG. 2, the capacitor C12 serves to adjust the inter- ^vention delay of the comparator 28 following the appearance of an overload (extinction of the phototransistor PT). This delay is necessary to allow the start of the circuit when it is turned on.

A problem arises in some devices (for example computer hard disk type or printhead) in which a transient or temporary current draw (for example, at startup) should not be interpreted as an overload. With a conventional circuit as shown in Figure 2, it is necessary to provide a capacitor C12 suf ^¬ important enough that it delays the detection so as not to trigger the comparator 28 during transient current calls. A disadvantage is that the entire circuit must be oversized to be able to withstand transient overloads. Indeed, the power supply will provide all the power required at the secondary level as long as the level remains below the protection threshold corresponding to the loss of secondary regulation. However, this level generally corresponds to a higher level than the temporary overloads that the circuit must accept for proper operation.

The present invention aims at providing a switching power control circuit that overcomes the disadvantages of known circuits.

The invention aims in particular to reduce the tripping threshold of a control circuit in case of overload compared to a conventional circuit, while allowing transient current spikes for starting the supplied loads. Summary of the invention

To achieve these objects as well as others, the present invention provides a detection circuit of a overload in a load powered by a switching power supply, comprising: a first comparator of a first voltage dependent on the supply voltage of the load with respect to a first threshold, providing a control signal to a pulse generator of control of the power supply ^¬ page; a second comparator of a second voltage with respect to a second threshold, providing a signal indicative of the presence of an overload; and means for controlling said second voltage to a third threshold and less than the second higher than the pre ^¬ Mier, and for deactivating the second comparator as this control is maintained. According to an embodiment of the present invention, said means comprise a third comparator of said second voltage with respect to the third threshold, supplying a control signal to an element bypassing a constant current source of charge of a capacitor connected to the mass and at the terminals of which said second voltage is measured.

According to one embodiment of the present invention, the circuit further comprises means for precharging said capacitor to said third threshold when the circuit is energized. According to one embodiment of the present invention, said second and first voltages are taken at the terminals of a dipole behaving as a current source whose value is a function of the voltage across the load.

According to one embodiment of the present invention, said dipole is a bipolar transistor.

According to one embodiment of the present invention, said transistor is an NPN type transistor whose emitter is connected to ground by a passive circuit that is at least resistive.

According to one embodiment of the present invention, the value of the passive circuit is chosen to output a current when the transistor is in the on state which is lower than the constant current supplied by the current source.

According to one embodiment of the present invention, the circuit further comprises means for forcing the first voltage to said third threshold when the circuit is energized, said passive circuit being a resistive and capacitive circuit.

According to one embodiment of the present invention, the transistor is a phototransistor of an optocoupler.

According to one embodiment of the present invention, the second comparator controls an element for supplying a control current to a pulse generating circuit. Brief description of the drawings

These and other objects, features, and advantages of the present invention will be set forth in detail in the following description of particular embodiments which is non-limiting in connection with the accompanying drawings, in which: FIG. described above represents the diagram of a switching power supply circuit of the type to which the present invention applies; Figure 2 shows the wiring diagram of a known control circuit; Figure 3 shows a control circuit according to an embodiment of the present invention; and FIGS. 4A and 4B illustrate, in the form of timing diagrams, the operation of the regulation circuit according to the invention. Detailed Description The same elements have been designated by the same references in the different figures. For the sake of clarity, only the elements that are necessary for understanding the invention have been shown in the figures and will be described later. In particular, the constituent details of the generating circuit of the control pulse trains of the

the switching switch have not been exposed, the invention being compatible with any conventional pulse train generation circuit, provided that it uses a control signal (error signal). Similarly, the secondary side measurement circuit of the switching power supply has not been exposed, the invention being again compatible with any conventional circuit provided that it provides, at a photodiode, a current function of the output voltage of the converter.

FIG. 3 represents, by a circuit diagram to be compared with that of FIG. 2, a regulation circuit 30 intended to provide an OVL signal for overload detection (terminal

21) and a control signal CT (terminal 26) to a conventional pulse train generation circuit (circuit 6, FIG.

1). As before, a first comparator 25 (in practice an operational amplifier mounted as a linear comparator) compares the voltage of an input terminal of a signal FB from the emitter of a phototransistor PT of an optocoupler (11). , FIG. 1) with respect to a reference voltage V _FB , and controls a MOS transistor M connected between the terminals 20 and 26.

According to the invention, a second comparator 31 receives, on an inverting input, a voltage VQVL setting an overload threshold, and has its output connected to the terminal 21. The non-inverting input of the comparator 31 is connected to a terminal 32 corre ^¬ ing to the collector of phototransistor PT optocoupler. The terminal 20 is also connected to the ground 24 by a resistor R, while the terminal 32 is connected to ground by a capacitor C33. If necessary, the resistor R is in series with another capacitor (not shown). Furthermore, the internal circuit 30, a source 34 of constant current BIAS ¹ ϋ ^e ^re ^a terminal 23 supply the voltage Vcc ALIMEN ^¬ tation continues to terminal 32 (non-inverting input of comparator 31). According to the invention, a third comparator 35 controls a switch M 35 (for example, a MOS transistor) bypassing the current source 34. The comparator 35 receives, on its non-inverting input, an initialization voltage V _INj , and its inverting input connected to the terminal 32. The role of the comparator 35 is to regulate the potential of the terminal 32, therefore the charge of the capacitor C33, to the voltage V _INj . The voltage ^V INI ^is chosen lower than the voltage V ^" OVL-

In normal operation, the voltage of the point 20 is regulated at the voltage V _FB by the conduction of the transistor M which therefore provides a reference current Ipg to the control circuit of the switching switch (not shown in FIG. 3). The current source 35 acts as the power source 27

(Figure 2), but inverted. In other words, instead of discharging the capacitor (C12, FIG. 1), it increases the charge of the capacitor C33 when the phototransistor is turned off.

The current IQ in the capacitor C33 is equal to ¹ BIAS ^-1 R ^-1 FB ' ^{or 1} R corresponds to the constant current (equal to V _FB / R) derived by the resistor R through the phototransistor PT.

As long as the sum of the two currents V _FB / R and Ipg is able to derive this constant current by taking it from the source 34, the comparator 35 manages to regulate the voltage of the terminal 32 and no OVL overload detection signal is transmitted to the circuit (6, FIG. 1) for generating the switching pulses. For this, the current supplied by the source 34 must be greater than the current V _FB / R + I _FB when the transistor PT is in normal operation.

The resistor R is chosen according to the desired rated power for the load without taking into account any inrush currents. Of course, in the dimensioning of the resistor R, it must be taken into account that the current flowing through it must remain lower than the current supplied by the source 34. Otherwise, a non-conducting operation is obtained. account for temporary surcharges. The Circuit of the invention is therefore versatile and can if necessary adapt to applications without problems of temporary overload. Alternatively, the resistive element R is adapted by an external command to allow overloads of longer duration.

In the event of a large current draw by the load, the current decrease in the phototransistor is such that it can no longer supply the resistor R the sum of the desired nominal currents. Surplus current is then taken to charge the capacitor C33. In particular, if the photo¬ transistor turns off, the current Iβ _S ^ e ^ - ^{a source} 34 is fully used to charge the capacitor C33.

The response time of the protection depends on the level of the overload. The lower the amplitude, the longer the triggering delay. The capacitor C33 is, for example, chosen to adjust approximately the allowed duration of the transient current calls (C = IT / Δu) with I corresponding to the current of the source 34 and Δu corresponding to

^V OVL ^"V INI- Thus, for transient overload, the comparator

31 does not have time to switch to change state OVL signal. The disappearance of the overload resets phototransistor PT and normal operation reappears, the charge of capacitor C33 is brought back to the level V _INj . By cons, if overcharging is sufficiently impor ^¬ aunt so that the voltage across the capacitor reaches the threshold voltage V _Q VL, the comparator 31, flip-flop.

To ensure the start circuit 30, an inter ^¬ breaker Kl connects the non-inverting input of comparator 35 at terminal 32. This switch Kl is closed upon starting of the circuit and is used to precharge the capacitor C33 to the voltage V _1NJ . Without this precharging, the collector voltage of the tran¬istor PT would remain lower than its emitter voltage and it could not be turned on. The voltage V _INI is chosen greater than the voltage V _FB so that the phototransistor PT is suitably polarized.

According to a preferred embodiment, a second switch K2 (optional) connects the non-inverting input of the comparator 35 to the terminal 20. This switch is closed at the same time as the switch K1 and allows a soft start (soft start) of the circuit if, in parallel, an RC network is externally connected to the terminal 20. In this case, when the preload is released, this RC network discharges into the terminal 20, which limits the output power. This actually generates a start ramp. According to an alternative embodiment, the resistor R is a variable resistor, which makes it possible to adjust the level of the operating threshold.

According to another variant, an additional current source injects a current directly into the resistor R, which makes it possible to lower the thresholds of the current peaks.

4A and 4B illustrate, in the form of chrono ^¬ grams, the operation of Figure 3. regulator 4A illustrates an example of shape of the power P supplied to the load, while Figure 4B illustrates the change of the voltage V32 on the terminal 32. In Figure 4B have been indicated the thresholds VJ _N J and VQVL, while a threshold PovL ^has been indicated in Figure 4A. This Povl threshold (φii depends on the value of the conden ^¬ sateur C33) is the power from which it is considered that there is a detected overload.

Initially, the level of power absorbed by the load is below the level PovL '^ ^a voltage V32 then remains at the level V _INj . It is assumed that at a time t1, a temporary overload occurs (for example, linked to a start of training of a computer hard disk). In this case, the power P increases abruptly and remains at a high level for a duration τ

(duration of the transient current peak). From time t1, the voltage V32 increases linearly but does not reach the threshold

VQVL ^at time t2 where the overload disappears. Consequently, no overload detection is transmitted to the circuit of control of the cutting switch. From time t2, if the normal operation is found, the voltage V32 _reaches the value V _INj gradually.

Suppose that at a time t3, another overload appears but this time with a greater intensity and lasting. As the intensity of the overload is greater, the power in the load is too. As a result, the charge of the capacitor C33 from the instant t3 is faster than from the instant t1. This is because the capacitor C33 is charged with a variable current

( ¹ BIAS ^-1 R ^-1 FB) • Depending on the Ipg current value, the capacitor

C33 receives more or less current. As soon as the voltage V32 reaches the threshold VQVL ' ^a signal indicating an overload is transmitted to the control circuit of the switching switch so that it opens the switch 5 permanently as the overload remains. From time t4 when the overload detection has been transmitted to the circuit 6, it can be considered that the extinction (in practice delayed) of the chopper switch 5 eliminates this overload and the power P decreases. In practice, the overload condition is memorized by the fact that the control loop requires even more power because the converter has stopped. This usually results in a general reset of the switching power supply and its restart (so-called hiccup mode).

The interpretation of the OVL overload signal is compatible with the usual uses of overload detection signals.

One advantage of the invention is that it avoids the classic oversized components of a ground fault cir ^¬ tion to accept switching transient surges, especially during operation. These components are here dimensioned according to the permissible power during such transient overloads. However, they do not have to sustain larger overcurrents permanently. the

Another advantage of the invention is that it maintains the regulation even during start-up current calls.

Indeed, as the overload detection (OVL signal) is not triggered, the circuit 6 receives a higher energy demand during these peaks, which is desirable.

In an embodiment in integrated form, the invention requires an additional terminal (32) to allow the connection of the collector of the optocoupler transistor PT.

Of course, the present invention is susceptible of various variations and modifications which will be apparent to those skilled in the art. In particular, the practical realization of the regulation circuit of the invention and in particular the dimensioning of the various components is within the abilities of those skilled in the art as a function of the application from the functional indications given above. In addition, although the invention has been described in connection with an example of preferred application to a switching power supply using an optocoupler for vehi ^¬ culate the control signal and an isolation transformer, it more generally applies, as soon as the control signal can be interpreted by the control circuit in the same way for an overload as for a normal additional current requirement and the load is likely to have transient do not constitute surcharges. For example, the same problem arises if the optocoupler is a bipolar transistor whose base is controlled by a voltage-current converter from a measurement of the voltage Vout. In addition, the bipolar transistor PT may consist of any dipole behaving as a current source whose value is used to drive the power element.

Claims

A circuit (30) for detecting an overload in a load supplied by a switching power supply, comprising: a first comparator (25) of a first voltage dependent on the supply voltage of the load with respect to a first threshold (V _FB ) providing a control signal (CT) to a generator (6) of control pulses of the switching power supply; a second comparator (31) of a second voltage with respect to a second threshold (VQVL) 'providing a signal (OVL) indicating the presence of an overload, characterized in that it comprises: means (C33, 34 , 35, M35) for slaving said second voltage ^¬ to a third threshold ( ^V INI) lower than the second and greater than the first, and for disabling the second comparator as long as this slaving is maintained.

2. Circuit according to claim 1, wherein said means comprise a third comparator (35) of said second voltage with respect to the third threshold (VJ _N J), provided ^¬ a control signal to a element (M35) bypassing a constant current source (34) for charging a capacitor (C33) connected to ground (24) and across which said second voltage is measured.

3. Circuit according to claim 2, further comprising means (K1) for precharging said capacitor (C33) to said third threshold (V _INI ) when the circuit is energized.

4. Circuit according to claim 1, wherein said second and first voltages are taken from the terminals (32, 20) of a dipole behaving as a current source whose value is a function of the voltage across the load.

The circuit of claim 4, wherein said dipole is a bipolar transistor (PT).

The circuit of claim 5, wherein said transistor (PT) is an NPN transistor whose transmitter is connected to the ground (24) by a passive circuit at least resistive (R).

The circuit of claim 6, wherein the value of the passive circuit (R) is selected to output a current, when the transistor (PT) is in the on state, which is less than the constant current supplied by the source. current (34).

8. Circuit according to claim 6, further comprising means (K2) for forcing the first voltage at said third threshold (V _INj ) to power up the circuit, said passive circuit being a resistive and capacitive circuit.

The circuit of claim 6, wherein the transistor is a phototransistor (PT) of an optocoupler (11).

10. Circuit according to claim 1, wherein the second comparator (25) controls an element (M) for supplying a control current (CT) to a pulse generating circuit.